Java并发编程：Callable&Future

Java并发编程：Callable&Future

在之前的线程中，执行完任务之后无法获取执行结果。

如果需要获取执行结果，就必须通过共享变量或者使用线程通信的方式来达到效果，这样使用起来就比较麻烦。

而自从Java 1.5开始，就提供了Callable和Future，通过它们可以在任务执行完毕之后得到任务执行结果。

用一个简单的例子说明用法：

import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;


public class CallableAndFuture{
	public static void main(String[] args) {
		ExecutorService threadPool = Executors.newSingleThreadExecutor();
		Future<String> future = 
		threadPool.submit(
			 new Callable<String>() {
				 public String call() throws Exception {
					Thread.sleep(2000);
					return "hello";
				 };
			 }
		);
		
		System.out.println("等待结果.......");
		try {
			System.out.println("等到结果：" + future.get());
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (ExecutionException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	/*
	 	CompletionService用于提交一组Callable任务，其take方法返回已完成的一个Callable任务对应的Future对象
	 	执行threadPool2池子中的任务
	*/
		ExecutorService threadPool2 = Executors.newFixedThreadPool(10);
		CompletionService<Integer> completionService = new ExecutorCompletionService<Integer>(threadPool2);
		
		for(int i=1; i<=10; i++){		

			final int seq = i;
			completionService.submit(new Callable<Integer>() {   
				@Override
				public Integer call() throws Exception {
					Thread.sleep(new Random().nextInt(5000));
					return seq;
				}
			});
		}
	
		for(int i=1; i<=10; i++){
			try {
				System.out.println(completionService.take().get());  //取出已经完成的任务
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (ExecutionException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}

}